1. Field of the Invention
The present invention relates to a method and related apparatus for catalytic steam cracking of hydrocarbons.
2. Description of the Prior Art
Steam cracking of hydrocarbons is one of the core processes in the petrochemical industry. Current world production of stream cracking products is estimated to reach 100 million metric tons/year of ethylene and propylene.
Basically, steam cracking comprises a step in which the hydrocarbon mixture to be transformed is mixed with steam and submitted to elevated temperatures in a tubular reactor, usually in the presence of one or more catalysts. The reaction temperature usually ranges from 700 to 900° C. according to the type of feedstock treated (the longer the hydrocarbon molecular structure, the lower the required temperature for cracking) while the residence time ranges from a few seconds to a fraction of second. The different resulting products, gaseous or liquid are then collected and separated. Thus, product distribution depends on the nature of the initial hydrocarbon mixture and the reaction conditions.
During steam cracking, light paraffins (ethane, propane and butane, obtained mainly by extraction from various natural gas sources) naphthas and other heavier petroleum cuts are broken down (cracked) into mainly:                i) light olefins: primarily ethylene and propylene,        ii) secondarily, depending on the feedstock employed, a C4 cut rich in butadienes and a C5+ cut with a high content of aromatics, particularly benzene,        iii) and finally hydrogen.        
Since enormous quantities of hydrocarbons are steam cracked throughout the world, even small yield or product selectivity improvements may lead to substantial commercial advantages.
Common feedstocks in steam cracking operations are ethane and LPG in the U.S.A. and naphthas or gas oils in Europe. However, in recent years, the situation has changed dramatically with the U.S.A. moving towards the use of heavier hydrocarbon feedstocks.
Market demands are currently focussed on propylene and on some longer isolefins such as isobutene and isopentenes. The latter enter in the synthesis of alkyl ethers used as octane boosters for transportation fuels. However, currently available steam cracking technology is not sufficiently flexible to respond to these or other market trends.
More than ten years ago, the present inventor had developed a method for upgrading the products of propane steam cracking, see U.S. Pat. No. 4,732,881. This process comprised adding a small catalytic reactor to a conventional propane steam cracker. The catalysts used were based on hybrid zeolite catalysts, namely ZSM5 zeolite modified with Al and Cr. Significant increases in the yield of ethylene and aromatics were obtained.
However, the prior art has so far partially failed to develop a method providing simultaneously:
(a) enhanced production of commercially valuable products light olefins (ethylene and propylene) and aromatics, and
(b) higher production flexibility and selectivity for olefins, i.e. a wider range of variation for the ethylene/propylene ratio.
It is therefore an object of the present invention to provide a novel method meeting these requirements.
Other objects and further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. It should be understood, however, that this detailed description, while indicating preferred embodiments of the invention, is given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art.